The world's population has exploded over the past century, growing from less than 2 billion to 7 billion people. And it's not stopping. The U.N.'s current projection is that humanity will number 9.3 billion individuals in 2050, and then hit 10.1 billion by 2100. Meanwhile, our energy resources are dwindling and droughts threaten our food supplies.

Have we reached a population crisis that will eventually destroy Homo sapiens entirely? How will we ever maintain our numbers at a sustainable size? The solutions to our population problem may be even more dangerous than the problem itself.

As fears about the energy and environmental crises reach a fever pitch, we're all searching for solutions. And one possibility is that we could fix everything if we'd just shrink our population back down to about 2 billion people — which would put us roughly where we were at 80 years ago. Alan Weisman, author of the influential environmental collapse book The World Without Us, has dedicated his next book, Countdown, to what he calls "the population crisis." He believes that humanity is headed to extinction if we can't deal with our species' growing numbers.

But Hampshire College developmental studies professor Betsy Hartmann and her colleagues believe that the population boom isn't the problem. She views this "crisis" as a red herring that distracts us from the real issues of "creating a sustainable solution" to humanity's basic needs for food, clean energy, medical care and education. She also worries that trying to control the birth rate will give governments an excuse to control women. Studies show that as more women around the world have access to education and birth control, they have fewer children. The population, Hartmann believes, will even out at about 10 billion — and that's an amount we can sustain.

So when it comes to the population explosion, there are two questions on the table. One, is our population growth going to kill us all? And two, is there any ethical way to prevent that from happening?

Back to the 1970s

The last time policy analysts and thinkers asked those questions in a sustained way was in the mid-twentieth century, after the Baby Boom. Tormented by fears of overpopulation leading to famine, a group of social scientists known as the Neo-Mathusians published a series of increasingly alarmist books about the coming "population bomb." In 1948, William Vogt sounded the alarm in his popular book Road to Survival. Like his muse, the British nineteenth century economist Thomas Malthus, Vogt believed that human population would be subject to "natural" checks like famine and disease. If we became too populous, we would head straight into disaster. Vogt later ran Planned Parenthood, hoping to curb population growth by educating people about birth control.

Around the same time Vogt published Road to Survival, a behavioral scientist named John B. Calhoun began a series of experiments on mice designed to reveal what would happen if a population kept growing unchecked, in a limited amount of space. Calhoun built a series of "mouse utopias," warm, roomy structures with lots of habitrail-like tunnels and dens in them, which he kept perpetually stocked with food. When the mouse population in them boomed, they eventually fell into cannibalism, senseless violence, sexual promiscuity or random celibacy, and finally lost their ability to socialize entirely. He called this turn to overpopulated anti-sociality "the behavioral sink," and this became a buzzword in the 1970s — it was sort of like the Singularity for dystopia, and spawned popular science fiction tales like the movie Soylent Green.

In the late 1960s, Neo-Malthusians William and Paul Paddock published the book Famine 1975! America's Decision: Who Will Survive?. This book predicted a behavioral sink in the wake of overpopulation, specifically as a result of starvation and food riots in the developing world. And it would all happen in the mid-1970s. Their solution? Wealthy, breadbasket nations like the U.S. would have to pick and choose what nations to aid — and leave some to die. They suggested a triage system, where the U.S. would "divide the underdeveloped nations into three categories," which they described like this:

1) Those so hopelessly headed for or in the grip of famine (whether because of overpopulation, agricultural insufficiency, or political ineptness) that our aid will be a waste; these "can't-be-saved nations" will be ignored and left to their fate; 2) Those who are suffering but who will stagger through without our aid, "the walking wounded"; and 3) Those who can be saved by our help.

Inspired by the the Paddocks, Paul and Louise Ehrlich followed up in 1968 with a book called The Population Bomb, which became a mega-bestseller in the early 1970s. Their book was written partly at the behest of then-director of the Sierra Club, David Brower. There was already a strong environmental component to these Neo-Malthusian doomsday scenarios. The Ehrlichs agreed with the Paddock's triage plan, suggesting at one point that India would be a ripe candidate for the "can't-be-saved-nation" starvation policy. They also suggested mass sterilization, a tax on children, and the creation of a population regulation agency like the FDA, but for family planning.

With suggestions like these, it's easy to see why some people were leery of the Neo-Malthusian idea. They suggested a very dystopian solution to our problems, and conjured up images of a future where America deliberately starved India to death, and the government coercively sterilized people by doping the water with birth control drugs. Plus, a new generation of technologies and food security strategies headed off that "Famine 1975!" scenario.

Entitlements and the Green Revolution

People like Vogt and the Ehrlichs were writing before the advent of what later came to be called the "green revolution," a toolset of agricultural technologies that allowed crop yields to double around the world in the 1970s and 80s. Using a combination of irrigation, fertilizer, pesticides, and GMO and specially-bred crops, farmers produced high-yield wheat and other staples, just managing to keep food supplies ahead of our growing population.

In the early 1980s, economist Amartya Sen produced a small tract called Poverty and Famines: An Essay on Entitlement and Deprivation, which offered a powerful new way of understanding how famine works. (Sen later won the Nobel Prize for this and other work.) In it, he explored how famine is not a "natural," Malthusian-style check on growing populations. Instead, it is a direct result of economic policies that deprive people of what he called entitlements, or access to food. Analyzing historical tragedies like the Irish Potato Famine, for example, reveals that bad economic and agricultural decisions led to the starvation of nearly a million Irish people. This was not nature at work. It was government and corporate decision-making that created economic inequality — which then led to famine.

Famine can be prevented, Sen argued, by paying attention to entitlement structures and monitoring when people lose those entitlements. Essentially, he suggested, we could deal with famine by mending our economic systems and preventing the kinds of abject poverty that killed a million in Ireland in the nineteenth century — and later killed millions more in the Bengal famine of the 1940s. Many demographers have since taken issue with Sen's analysis, but nevertheless he changed the way we understand famine by turning it into an entitlement issue, rather than a Malthusian debate over nature.

Many studies have shown that more educated women have fewer children, but researchers have never…
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As Hartmann said via email:

Moreover, while family planning can help facilitate the transition to lower birth rates, it is rarely the motor force behind the demographic transition. Instead it is major social and economic changes, education, women's employment, the changing value of children, etc., that changes people's desired family size. Urbanization is also key.

Like Sen, Hartmann believes we've defused the population bomb, and we've done it with social changes that stop the extreme deprivation of groups like women and the poor.

The Return of the Bomb

Despite all these social changes, the question remains: Is it healthy for humanity to number over 10 billion on a finite planet? Especially when peak oil is coming faster than ever, and the tools of the green revolution (AKA factory farming) may be catalyzing climate change?

Some, like author Alan Weisman, believe we must lower the population or the world will have to go on without us. His forthcoming book Countdown will explore how we'll do that. A recent TED talk, by actress Alexandra Paul, advocated a goal of "one child per couple" or none. Her goal is to get the world's population back down to 2 billion people, roughly where it was in 1930.

On the population side, it is clear that avoiding collapse would be a lot easier if humanity could entrain a gradual population decline toward an optimal number. Our group's analysis of what that optimum population size might be like comes up with 1.5 to 2 billion, less than one third of what it is today. We attempted to find a number that would maximize human options – enough people to have large, exciting cities and still maintain substantial tracts of wilderness for the enjoyment of outdoors enthusiasts and hermits (Daily et al. 1994). Even more important would be the ability to maintain sustainable agricultural systems and the crucial life support services from natural ecosystems that humanity is so dependent upon.

Meanwhile, our pop culture is full of Malthusian fears about famine and disease. The Hunger Games and The Walking Dead present post-apocalyptic worlds where humanity's growing numbers have been "checked" in the most grisly ways possible.

Still, Hartmann believes our population can be sustained even if we add 2-3 billion people to the planet. She writes:

The real challenge that lies ahead is how to plan for the addition of 2-3 billion additional people in environmentally sustainable and socially equitable ways. It can be done, but it will take a lot of ingenuity, innovation, and above all, political will. What doesn't help is getting caught in the apocalyptic Malthusian trap that the planet cannot possibly support that many people. Barring major catastrophes – thermonuclear war, an asteroid strike, the plague of all plagues – it will have to. The question is not if, but how.

Most importantly, many of the methods we've explored to shrink our population are in fact uglier than dealing with 10 billion people. Forced sterilization? Starving India? These are not good options.

Instead, it might be better to invest in education. Not just for women who need information on birth control, but for everyone who needs to learn quickly how to create sustainable agriculture and energy for a population of 10 billion. Instead of trying to turn back the clock, we need to move forward. We can't control or starve certain groups in order to return to the 1930s.

Calhoun, the scientist who coined the term behavioral sink, would agree. Though his work with mice convinced him that overpopulation would lead to social collapse, he never advocated population control as a solution. Instead, he spend a great deal of time toward the end of his life trying to promote space colonization. He wanted to see humanity spread out, find more places to live, and learn to survive while also expanding.

Maybe the problem isn't our population size. Instead, the problem may be with how we've chosen to deal with it.